I've been playing around with the ARTA audio measurement software from https://artalabs.hr/ 

Although intended primarily for loudspeaker measurements, it does a very good job for tube amplifiers as well. I use this DAC https://focusrite.com/clarett-plus

Here are some spectrum measurements of the SE5 amplifier (UL mode, Standard Transformer, Feedback ON, 3Watts RMS output).

First pic: Noise floor with SE5 on but no Bluetooth module installed. There is a small peak at 50Hz but noise is extremely low and mostly due to instrumentation noise floor and pick-up from cables etc.

Second pic: Same as first measurement but with Bluetooth module installed. You can see some peaks from the Bluetooth module internal oscillators, but its still very low level and well below anything audible.

Third pic: 3Watts RMS into an 8Ohm load. Bluetooth module not inserted. Second harmonic distortion is predominant, and then all higher-order harmonics are descending in level. This is the type of harmonic structure that audiophiles look for in a single-ended amplifier, and what makes them sound so good.